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Classification of Stress Action.

The quantities in this table are given in tons, because the
numbers are thus more easily remembered, and because it is the
Engineer's language. Fig. 350 shews them diagrammatically.

(See Appendicesi pp. 840 and 1075 for further materials.)

The Proportioning of Structures and Machine
Parts by Calculation.—The equality of action and reaction
is the starting point in constructive calculation. Whether the
load be applied directly or through a lever arm, the external
forces must balance the internal stresses, and we have for the two

(Direct action).    Total load = Total stresses.

(Lever action).    Moment of load = Moment of stresses,*

which are our general strength equations.

Classification of Stress Action. — Practical cases of
simple or compound stress may be arranged under ten

* A moment = force x lever arm.